This study will characterize genetically and biochemically the unique lexC mutations that regulate enzymatic activities (SOS functions) necessary for the completion of postreplication repair, the induction of error-prone processes, the induction of bacteriophage, and the regulation of cell division by a septation inhibitor. The physiological interaction of the lexC-113 mutation and two putative lexC mutations with lexA, tsl(lexA), tif, and recA mutations shall be ascertained by constructing multiple mutants and determining for comparison the expression of the above SOS functions, the sensitivity to colicins and the sensitivity to repair-blocking chemicals. The constitutive and induced synthesis of protein X in the above strains will be determined by two dimensional electrophoresis to establish which of two proposed models for the regulation of SOS functions by the lexC gene is incorrect. Rates of synthesis and degradation of protein X in the above strains and in lon and sul derivatives shall test predictions arising from the Satta-Pardee model for the regulation of septum formation and elucidate roles for the lon and sul genes. The protein(s) synthesized by the mutant Pl bacteriophage, PlCmlxc, that substitutes for the function absent in lexC mutants shall be characterized by two dimensional electrophoresis to circumscribe the number of functions associated with the lxc mutation. The effect of PlCmlxc bacteriophage upon the synthesis of bacterial proteins induced after exposure of wild type and mutant cells to SOS inducing stimuli shall be determined to illuminate the mechanism of suppression. Physiological and biochemical studies of lambda induction in tif mutants with the mutant PlCmlxc bacteriophage will test a model that hypothesizes that protein X modifies regulatory proteins, sensitizing them to attack by the lexC analog, a protease. Sensitive mutagenesis studies with PlCmlxc in LexA, tsl(lexA), lexC, and recA strains will determine the extent of interaction between the lexC analog and lexA and recA gene products.